Analog-to-digital converting apparatus having an automatic gain control circuit

ABSTRACT

An analog-to-digital converting apparatus includes an analog controllable gain amplifier (A 1 ) connected to an input of an analog-to-digital converter A/N. A gain control signal of the amplifier (A 1 ) is obtained from at least one comparator (D16, D240, D224) for making a binary comparison between an output level of the analog-to-digital converter (A/N) and at least one predetermined threshold, and circuitry (SEL2, S 2 , I 2 ) for controlling with respect to time the gain of the amplifier (A 1 ) as a function of the result of the comparison.

BACKGROUND OF THE INVENTION

The present invention relates to an analog-to-digital converting apparatus having an automatic gain control circuit comprising an analog gain controllable amplifier and an analog-to-digital (A/D) converter coupled to an output of the analog amplifier, a gain control loop of said analog amplifier receiving output signals from the A/D converter.

Such an analog-to-digital converting apparatus is known from the French Patent Application No. 2 536 620 (RCA Corp.), which corresponds to U.S. Pat. No. 4,517,586. Herein, the values of the digital samples from the A/D converter are digitally compared to a reference level, in order to produce a digital difference signal which is converted by a digital-to-analog converter into an analog gain control signal of the gain controllable amplifier. This solution leads to a complicated and thus costly circuitry.

SUMMARY OF THE INVENTION

The present invention has for its object to provide an analog-to-digital converting apparatus of the type stated before, in which no digital-to-analog converter is utilized for controlling the amplifier gain. Hence, the A/D converting apparatus is simplified and thus less costly.

To this end, the analog-to-digital converting apparatus according to the invention is characterized in that it comprises at least one comparator for making a binary comparison between an output level of the analog-to-digital converter and at least one given threshold, and means for controlling the amplifier gain in a time-controlled fashion as a function of a result of said comparison.

The above comparator comprises advantageously a decoder of a logic level corresponding to at least one white level of at least one television transmission system.

The means for controlling the amplifier gain in a time-controlled fashion can comprise a first controllable current source effecting the charging and discharging of a first capacitor, which is coupled to a gain control input of said amplifier.

The above first controllable current source can comprise a first differential stage having a first and a second transistor whose emitters are coupled to a first control current source, the collector of the first transistor being coupled to a supply voltage source and that of the second transistor to said first capacitor as well as a second control current source, the bases of the first and second transistors receiving a first differential signal corresponding to the result of said comparison, the first control current source having a current value distinctly superior to that of the second control current source, so that the first capacitor is charged with a given time constant across the second control current source for a first logic state of the first differential signal, and for a second logic state of the said signal, said first capacitor is discharged with a shorter time constant across the first control current source.

The A/D converting apparatus according to the invention can also comprise means for controlling, in a time-controlled fashion, a continuous level at a signal input of the amplifier as a function of the result of a comparison produced by a second decoder of a logic level, corresponding to a continuous level standardized by at least one television transmission system. Said means for controlling the continuous level in a time-controlled fashion can also comprise a second controllable current source effecting the charging and discharging of a second capacitor, which is coupled in series to the signal input of said amplifier.

In a preferred embodiment, the second controllable current source comprises a second differential stage having a third and a fourth transistor whose emitters are coupled to a third control current source, the collector of the third transistor being coupled to that of a fifth transistor whose emitter is coupled to said supply voltage source, the collector of the fourth transistor being coupled to said second capacitor as well as the collector of a sixth transistor whose emitter is coupled to said supply voltage source, the bases of the fifth and sixth transistors being intercoupled, and the base and the collector of the fifth transistor being coupled so that the fifth and sixth transistors form a current mirror circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood when reading the description given hereinbelow as a non-limiting example, with reference to the following Figures, in which:

FIG. 1 shows a general diagram of an A/D converting apparatus according to the invention;

FIG. 2 shows a diagram of an embodiment of automatic gain correcting logic in universal television receivers;

FIG. 3 shows a diagram of an embodiment of continuous level correcting logic in universal television receivers;

FIGS. 4 and 5 show embodiments of the controllable current sources corresponding to an automatic gain control and a continuous level correction, respectively;

FIGS. 6, 7a and 7b show, respectively, PAL or SECAM luminance signals, a signal according to the MAC television transmission system, and a detail of a luminance signal of last lines of a picture according to the MAC television transmission system;

FIG. 8 shows logic stages of a GRAY encoding provided by outputs G₁ . . . G₈ of the A/D converters;

FIG. 9 shows a decoder operating on the basis of this code; and

FIGS. 10 and 11 show an embodiment of a circuit producing signals S₂ and S₁ of the respective FIGS. 2 and 3 from outputs of the decoder of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

According to FIG. 1, a gain control input of an amplifier A₁ of the variable gain type is connected to one of two terminals of a capacitor C₂, whose other terminal is connected to ground.

The charging and discharging of the capacitor C₂ is obtained from a controllable current source I₂ which allows the realization of a counter reaction of automatic gain control. An output of the amplifier A₁ drives an analog-to-digital (A/D) converter A/N having outputs S which produce a digital signal whose level is to be controlled according to certain parameters, more specifically, in order to avoid saturation at the output of the A/D converter A/N. In this specific case, an analog-to-digital converting apparatus is shown for a television receiver which can be used for the classical PAL and SECAM reception as well as for satellite reception according to the MAC television transmission system. Thereto, the signals S are applied to decoders D₁₆, D₂₄₀ and D₂₂₄ whose output signals s₁₆, s₂₄₀ (for the MAC television transmission system) and s₂₂₄ (PAL or SECAM) each have a logic 1 or 0 level depending on the condition of the output signals S of the A/D converter A/N. The signals s₁₆, s₂₄₀ and s₂₂₄ are applied to a selecting circuit SEL2, which receives a logic selecting signal S₀ indicating whether the receiver operates on a transmission according to the PAL or SECAM television transmission system, or a transmission according to the MAC television transmission system. The selecting circuit SEL2 also receives control pulses p₂ of the automatic gain control.

The output signal S₂ of the selecting circuit SEL2 controls a controllable current source I₂ in one direction or the other with an intensity which can vary with the direction, in order to attain a time-controllable charging or discharging of the capacitor C₂, with time constants which may be different.

According to the invention, a correction of a continuous level can also be realized in an analog manner by controlling a time-controlled charging and discharging of a capacitor C₁ connected in series to a signal input of the amplifier A₁ with the aid of a controllable current source I₁.

While reference is always made to the preceding example, concerning a universal television receiver, the output signals S of the analog-to-digital converter A/N are applied to the inputs of the two decoders D₃₂ and D₁₂₈ producing at their outputs, the signals s₃₂ (PAL or SECAM) and s₁₂₈ (for the MAC standard) each having a logic 1 or 0 level depending on the value of the output signals S. The signals s₃₂ and s₁₂₈ are applied to a selecting circuit SEL1 receiving the logic selecting signal S₀ indicating whether the receiver operates according to a PAL or SECAM television transmission system or otherwise according to the MAC television transmission system. The controllable current source I₁ also receives control pulses p1 to correct the continuous level (see FIG. 6). Such a pulse is present in the PAL or SECAM television transmission system at the beginning of each line at the moment when the signal has reached a level equal to one-eighth of the maximum amplitude (32 for an 8-bit encoding). According to the MAC television transmission system, the signal p1 is a pulse 750 ns in length at the beginning of each line during which the signal has a level equal to one-half the maximum amplitude, which is 128 for 8-encoding (see FIG. 7a). The output signal S₁ of the selecting circuit SEL1 controls the current source I₁ in one direction or in the other in order to attain a time-controlled charging or discharging of the capacitor C₁.

With reference to FIG. 2, it will now be described in a more precise way how the automatic gain control signal S₂ is generated. According to the MAC television transmission system, the correction takes place when a signal occurs produced on the line 624 of each picture (see FIG. 7b), which is reversed to the white, black and grey reference signals. As the signal is symmetrical, automatic gain control is achieved controlling the white level as well as the black level. The black level is obtained when decoding the 16-value and the white level by decoding the 240 value. Owing to the symmetry of the reference signal of line 624 (MAC television transmission system), the automatic gain control is controlled by a signal s₁₆ which has, in the present case, the 1-value when the output of the analog-to-digital converter A/N has a value of ≦15 and the 0-value if this is not the case, and by a signal s₂₄₀ which has the 1-value when the output of the A/D converter has a value of ≧240 and 0 if this is not the case. The signal s₂₄₀ is an output signal of an AND gate 20, four inputs of which receive the four high-order bits from the output of the converter, that is to say Q₅, Q₆, Q₇ and Q₈, respectively. The signal s₁₆ is an output signal of an inverting OR gate 21, four inputs of which also receive the bits Q₅, Q₆, Q₇ and Q₈, respectively.

The signals s₁₆ and s₂₄₀ are applied to two inputs of an OR gate 22, an output signal of which forms a signal s₁₇ and drives one of the two inputs of an AND gate 23 whose other input receives the pulses p2 representative of the line 624 (see FIG. 7b). An output of the AND gate 23 is applied to an input of an AND gate 24, a further input of which receives the selecting signal S₀ inverted by an invertor 14.

According to the PAL or SECAM television transmission systems, the reference level for realizing the automatic gain control is the level 224 from the output of the A/D converter. In contradistinction to the MAC television transmission system, the automatic gain control is realized line by line. The signal s₂₂₄ is chosen so as to have a logic 1 level for an A/D converter output level ≧224, and a logic 0 level if this is not the case. In order to achieve this, the three highest order bits Q₆, Q₇ and Q₈, respectively, are applied to three inputs of an AND gate 25, an output of which, supplying signal s₂₂₄, drives one of two inputs of an AND gate 26 whose other input receives the selecting signal S₀. Outputs of the AND gates 24 and 26 are applied to respective inputs of an OR gate 27, an output of which supplies the signal S₂.

With reference to FIG. 3, it will now be described in a more precise manner how the continuous level correcting signal S₁ is generated. In the PAL or SECAM television transmission systems, the black background level corresponds to the 32 value at the output of the 8-bit analog-to-digital converter A/N. The 32 value is attained when one of the three first high order bits Q₆, Q₇, Q₈ has the value of 1. Consequently, the detection (decoder D₃₂) of the continuous reference background level is realized by connecting the outputs Q₆, Q₇ and Q₈ to inputs of the respective invertors 1, 2 and 3, outputs of which drive three inputs of an inverting AND gate 10. Thus, the output signal s₃₂ has the 0 level when Q₆ =Q₇ =Q₈ =0 and the 1 level in all the other cases, that is to say, as soon as the black background level reaches the digital 32 value. In the MAC television transmission system, the correction of the continuous level is not effected on the black background level but on the grey level which corresponds to a value of 128 at the output of an 8-bit analog-to-digital converter. For the decoder (decoder D₁₂₈). it will be sufficient to take the high-order output Q₈ of the converter, producing directly the signal s₁₂₈. s₁₂₈ has the 1 level as soon as the grey level reaches the 128 value. The signal S₃₂ is applied to one of two inputs of an AND gate 11 whose other input receives a logic signal S₀, while S₀ has the logic 0 level when the reception takes place according to the MAC television transmission system and the logic 1 level for the PAL and SECAM television transmission systems. The signal s₁₂₈ is applied to one of two inputs of an AND gate 12, whose other input receives the logic signal S₀ reversed by an inverter 4. Outputs of the AND gates 11 and 12 are connected to inputs of an OR gate 13 delivering at an output the control signal S₁ for the current source I₁. When S₀ =0 (MAC television transmission system) we have S₁ =s₁₂₈ ; and S₁ =s₃₂ when S₀ =1 (PAL or SECAM television transmission systems). The authorization signal p₁ appears at the beginning of each line by way of a 750 ns pulse for the MAC television transmission system and a pulse of 4 microseconds in the PAL or SECAM television transmission systems and is utilized for authorizing the control of the current source I₁ as will be discussed hereinafter.

FIG. 4 shows the operation of the automatic gain control from signal S₂ onwards. The signal S₂ is preferably a differential signal whose high level and low level are for example 200 mV apart, so as to drive the bases of transistors T₁₀ and T₁₁ constituting the inputs of a differential stage (signals e₁₀ and e₁₁). The emitters of transistors T₁₀ and T₁₁ are interconnected and connected to a current source I₁₂. The collector of transistor T₁₀ is directly connected to a positive supply voltage source +V, while that of the transistor T₁₁ is connected to a current source I₁₁. For example, when S₂ has a high level, the respective values of 2.2 V and 2 V can be obtained for e₁₀ and e₁₁ and vice versa for a low-level S₂. The current source values of I₁₁ and I₁₂ are chosen differently so as to obtain different time constants for charging and discharging the capacitor C₂.

A low value is chosen for the value of the current source I₁₁ in order to obtain a slow charging of the capacitor C₂, for example, with a time constant t₁ of a few seconds, and a value which is distinctly higher for the current source I₁₂ so as to obtain a rapid discharging of the capacitor C₂ while avoiding complete saturation, for example, with a time constant t₂ of 500 microseconds. By way of example, I₁₁ =100 nA and I₁₂ =500 μA are possible for a capacitor C₂ having a value of 0.47 μF.

When the signal S₂ has the logic 1 value, e₁₁ >e₁₀, the transistor T₁₁ is conducting and the transistor T₁₀ is non-conducting. The capacitor C₂ is rapidly discharged owing to the current source I₁₂ because its current exceeds by far that of the current source I₁₁.

When the signal S₂ has the logic 0 level, e₁₀ >e₁₁, the transistor T₁₀ is then in the conducting state and the transistor T₁₁ in the non-conducting state. The capacitor C₂ is slowly charged due to the low current value produced by the current source I₁₁.

It should be noted, by way of variant, that the differential stage constituted by the transistors T₁₀ and T₁₁ can be replaced by two differential stages in a cascade arrangement.

FIG. 5 shows how the signal S₁ is utilized for controlling the current source I₁. The signal S₁ is preferably a differential signal of which the high and low levels are 200 mV apart, for example, so as to drive the bases of the transistors T₁ and T₂ of a differential stage. For example, for the 1 level, the values of the signals e₁ and e₂ applied to the respective bases of the transistors T₁ and T₂ are 2.5 and 2.7 V, respectively, and 2.7 and 2.5 V, respectively, for the 0 level. The interconnected emitters of the transistors T₁ and T₂ are connected to the collector of a transistor T₄ whose emitter is connected to ground and whose base is connected to that of a transistor T₃, forming therewith a current mirror circuit, whose emitter is connected to ground, whose base and collector are interconnected and which receives pulse p₁ across a resistor R₃ , so as to enable a correction of the continuous level only during a planned time interval.

Two transistors T₅ and T₆ are arranged in a current mirror circuit with the collectors of transistors T₁ and T₂. The emitters of these transistors T₅ and T₆ are connected to a supply voltage source +V (for example 5 V), their bases are interconnected and their collectors are connected to those of transistors T₁ and T₂. Furthermore, the base and the collector of the transistor T₅ can be directly interconnected or also, as shown, through the emitter-base path of a transistor T₇ whose collector is connected to ground. If one of the two signals selected by S₀, S₃₂ or s₁₂₈, has the 1 level, then e₂ >e₁, and the transistor T₂ is conducting while the transistor T₁ is non-conducting, and a discharging of the capacitor C₁ is achieved through transistor T₄ operating as the current source for the duration of the pulse p₁ which lowers a DC current level of the signal input of the amplifier A₁. In contradistinction thereto, if one of the two selected signals s₃₂ or s₁₂₈ has the 0 level, then e₁ >e₂, and the transistor T₁ is conducting while the transistor T₂ is non-conducting. As the transistors T₅ and T₆ constitute a current mirror circuit, the capacitor C₁ is charged by the current of the same intensity as that passing through transistor T₄ and which passes through the transistor T₆ for the duration of the pulse p₁, which makes the DC current level at the signal input of the amplifier A₁ rise again.

FIG. 8 shows the logic levels of the four high order outputs in the GRAY code of the analog-to-digital converter. It will be remembered that the GRAY code is a code for which any incrementation by a positive or negative unit of a number only changes one of the bits. In this special case, the GRAY code is chosen for its property of simplifying the logic for the logic levels to be processed.

A G₅ bit has the 0 level for the logic values between 0 and 15, and changes to the 1 level for the 16 value. It changes again each time a multiple of 32 is added to the 16 value, the last change, a return to zero, taking place for the 240 value. A G₆ bit has the 0 level for the logic values between 0 and 31 and changes to the 1 level for the 32 value. It changes again each time a multiple of 64 is added to the 32 value, the last change, a return to zero, taking place for the 224 value. A G₇ bit has the 0 level for the logic values between 0 and 63 and changes to the 1 level for the 64 value and returns to the zero level for the values superior to 192. A G₈ bit has the 0 level for the logic values between 0 and 127, and to the 1 level for the logic values between 128 and 256.

The signal s₁₇ must have the 0 level for the values between 0 and 15 and 240 and 256, and the 1 level if this is not the case. The latter happens when G₅ =G₆ =G₇ =0. The signal s₂₂₄ has the 0 level for the values between 0 and 223 and the 1 level for those comprised between 224 and 256. The latter happens when G₆ =G₇ =0 and G₈ =1. The signal s₃₂ has the 0 level for the values comprised between 0 and 31 and the 1 level for those comprised between 32 and 256. The latter happens when G₆ =G₇ =G₈ =0.

The signal s₁₂₈ finally corresponds directly to the logic output G₈.

According to FIG. 9, the decoding of the stages discussed hereinbefore is realized from logic outputs G₅ to G₈ and their complements available in a logic TTL form. The general organization of the decoder consists in differential transmitter pairs coupled and spread out over three levels, with two differential pairs per level and switching current sources. The signals G₅, G₆, G₇ and G₈ are applied to the base of the respective transistors T₁₅, T₁₆, T₁₇, T₁₈, T'₁₅, T'₁₆, T'₁₇ and T'₁₈ whose collectors are connected to a supply voltage V, having the value of 5 V.

Eleven transistors T₆₀ to T₇₀ form eleven current sources and each receives on it base a same reference voltage V', and their emitters are connected to ground. The collectors of transistors T₆₀ and T₇₀ are connected to the emitters of respective transistors T₁₅ and T'₁₅ across three level shifting diodes arranged in series and in the forward direction. The collectors of transistors T₆₁ and T₆₉ are connected to the respective emitters of the transistors T₁₆ and T'₁₆ across two level shifting diodes arranged in series and in the forward direction. The collectors of the transistors T₆₂ and T₆₈ are connected to the respective emitters of the transistors T₁₇ and T'₁₇ across one level shifting diode arranged in the forward direction. The collectors of transistors T₆₃ and T₆₇ are connected directly to the emitters of the respective transistors T₁₈ and T'₁₈.

The two differential pairs of each level are constituted by emitter-coupled transistors T₃₀, T₃₁ and T₃₂, T₃₃ for the first level, T₄₀, T₄₁ and T₄₂, T₄₃ for the second level and T₅₀, T₅₁ and T₅₂, T₅₃ for the third level. The interconnected emitters of the transistors T₃₀ and T₃₁, T₃₂ and T₃₃, T₄₀ and T₄₁, T₄₂ and T₄₃, T₅₀ and T₅₁, and T₅₂ and T₅₃ are connected to the collectors of the respective transistors T₄₁, T₄₃, T₅₁, T₅₃, T₆₄ and T₆₅. The collectors of the transistors T₃₀, T₄₀ and T₅₀ are interconnected and connected to the base of a transistor T₂₀, which is further connected to the supply voltage V via a resistor R. The collector of the transistor T₂₀ is connected to the supply voltage V. The collector of the transistor T₃₁ is connected to the base of a transistor T₂₁, which is further connected to the supply voltage V via a resistor R. The collector of the transistor T₂₁ is connected to the supply voltage V. The collectors of the transistors T₃₂ and T₃₃ are connected to the bases of respective transistors T₂₂ and T₂₃, which are further connected to the supply voltage V via respective two resistors R. The collectors of the transistors T₄₂ and T₅₂ are connected to the supply voltage source V.

The collectors of the transistors T₆₀ to T₆₃ are connected to the bases of the respective transistors T₅₀ ; T₄₀ and T₅₂ ; T₃₀ and T₄₂ ; and finally T₃₂. The collectors of the transistors T₆₇ to T₇₀ are connected to the bases of the respective transistors T₃₃ ; T₃₁ and T₃₃ ; T₄₁ and T₅₃ ; and finally T₅₁. The base of a transistor T₂₄ is connected to the collector of transistor T₆₆ and to the supply voltage V via a resistor having the value of R, its collector also being connected to the supply voltage V.

The signal s₁₇ is available between the emitters of the transistors T₂₀ and T₂₁. This signal s₁₇ is a combination of two signals s₂₀ and s₂₁, available at these emitters. As a matter of fact, when G₅ =G₆ =G₇ =0, the transistors T₃₁, T₄₁ and T₅₁ are conducting and the transistors T₃₀, T₄₀ and T₅₀ are non-conducting. The signal s₂₂₄ is available between the emitters of the transistors T₂₂ and T₂₄. This signal s₂₂₄ is a combination of two signals s₂₂ and s₂₄, available at these emitters. When G₆ =G₇ =0 and G₈ =1, the transistors T₃₂, T₄₃ and T₅₃ are conducting.

The signal s₃₂ is available between the emitters of the transistors T₂₃ and T₂₄. This signal s₃₂ is a combination of two signals s₂₃ and s₂₄ available at these emitters.

When G₆ =G₇ =G₈ =0, the transistors T₃₇, T₄₃ and T₅₃ are conducting.

Finally, the signal s₁₂₈ is directly available between the emitters of the transistors T₁₈ and T'₁₈. This signal s₁₂₈ is a combination of two signals s₁₈ and s'₁₈ available at the emitters of these transistors.

As shown in FIG. 10, an output signal s'₂ is obtained from the signals s₂₀, s₂₁, s₂₂, s₂₄, p₂ and S₀.

Two differential pairs, constituted by emitter-coupled transistors T₇₁ and T₇₂, T₇₃ and T₇₄ respectively, receiving on their bases the respective signals s₂₀, s₂₁, s₂₂ and s₂₄, form a control level. The collectors of the transistors T₇₁ and T₇₃ are interconnected and coupled to the supply voltage V via a resistor R'. Also, the collectors of the transistors T₇₂ and T₇₄ are interconnected and coupled to the supply voltage V via a resistor R'.

A first switching level is formed by a differential pair comprising two emitter-coupled transistors T₇₅ and T₇₆ receiving on their bases the signal p₂ and a reference voltage signal V_(REF).sbsb.1, respectively. The collector of the transistor T₇₅ is connected to the coupled emitters of the transistors T₇₁ and T₇₂, while the collector of the transistor T₇₆ is connected to the interconnected collectors of the transistors T₇₁ and T₇₃.

A second switching level is formed by a differential pair comprising two emitter-coupled transistors T₇₇ and T₇₈ receiving on their respective bases a reference voltage signal V_(REF).sbsb.2 and the signal S₀ level-shifted by a diode voltage.

The collectors of the transistors T₇₇ and T₇₈ are connected to the coupled emitters of the transistor T₇₅ and T₇₆, on the one hand, and T₇₃ and T₇₄, on the other hand, respectively. A current source I₄ referenced to earth is connected to the coupled emitters of the transistors T₇₇ and T₇₈.

When S₀ has a high level (operation according to the PAL or SECAM television transmission system); the transistor T₇₈ is conducting and the output signal s'₂ corresponds to the logic state of the signal s₂₂₄ (signals s₂₂ and s₂₄). When S₀ has a low level, the transistor T₇₇ is conducting and the output signal s'₂ corresponds to the logic state of the signal s₁₇ (signal s₂₀ and s₂₂). When the signal p₂ has the 1 level, the transistor T₇₅ is conducting. When the signal p₂ has the 0 level, the transistor T₇₆ is conducting and the collector of the transmitter T₇₁ retains the low level whatever logic state the inputs s₂₀ and s₂₁ have, and the signal s'₂ is kept at the 0 level, producing a slow charging of the capacitor C₂ by the current source I₁₁ (FIG. 4).

To obtain the signal S₂, from the signal s'₂ the signals present at the collectors of the transistors T₇₁ and T₇₂ are applied to the bases of respective transistors T₇₉ and T₈₀ inserted as emitter-followers, whose collectors are connected to the voltage source V, and whose emitters are coupled to ground via resistors R", to whose terminals are connected capacitors C" in parallel and respective current sources J₁ and J₂, the points A and B denoting the terminals common to the resistors R" and the current source J₁ and J₂, respectively.

In the couplings of collectors of the transistors T₇₁ and T₇₂ to the supply voltage V, resistors R' are used for adjusting the differential voltage of S₂. For example, with R'=1.6 kΩ and I₁ =150 μA, a differential voltage of 240 mV is obtained.

Assuming that V_(A) and V_(B) are the voltages at points A and B,

    V.sub.A =V-V.sub.be -R"J.sub.1 -R'I.sub.1 k

    V.sub.B =V-V.sub.be -R"J.sub.2 -R'I.sub.1 (1-k)

in which V_(be) denotes the base-emitter voltage drop of a transistor and k has the 0 or 1 value according to the switching direction of the differential pairs. When V=5 Volts, R"=9.7 kΩ, J₁ =J₂ 100 μA, R'=1.6 kΩ I₁ =150 μA and V_(be) =0.8 V we obtain:

    for k=0 V.sub.A =2.4648 V V.sub.b =2.408 V

    for k=1 V.sub.A =2.408 V V.sub.b =2.648 V.

As shown in FIG. 11, the output signal s'₁ is obtained from the signals s₂₃, s₂₄, s₁₈, s'₁₈ and S₀.

Two differential pairs constituted by emitter-coupled transistors T₈₁ and T₈₂, T₈₃ and T₈₄, respectively, receiving on their bases the respective signals s₂₃ and s₂₄ (together s₃₂), on the one hand, and s₁₈ and s'₁₈ (together s₁₂₈), form a control level. The collectors of the transistors T₈₁ and T₈₂, on the one hand, and T₈₂ and T₈₄, on the other, are interconnected and coupled to the supply voltage V via resistors R'.

A switching level is constituted by a differential pair comprising two emitter-coupled transistors T₈₅ and T₈₆ receiving on their bases the respective signal S₀ and reference voltage signal V_(REF).sbsb.2. The collector of transistor T₈₅ is connected to the interconnected emitters of the transistors T₈₁ and T₈₂, while the collector of the transistor T₈₆ is connected to the interconnected emitters of transistors T₈₃ and T₈₄. A current source I₃, which has a first terminal being connected to ground, has a second terminal connected to the interconnected emitters of the transistors T₈₅ and T₈₆. When S₀ has a high level (operation according to the PAL or SECAM television transmission system), the transistor T₈₅ is conducting and the output signal S_(i) corresponds to the logic state of the signal s₃₂ (signals s₂₃ and s₂₄). When S₀ has a low level (operation according to the MAC standard), the transistor T₈₆ is conducting and the output signal s₁ corresponds to the logic state of the signal s₁₂₈ (signals s₁₈ and s'₁₈).

The signal S₁ can be obtained from the signal S'₁ in the same manner as the signal S₂ is obtained from the signal S'₂. 

What is claimed is:
 1. An analog-to-digital converting apparatus having an automatic gain control circuit comprising an analog gain controllable amplifier and an analog-to-digital (A/D) converter coupled to an output of said analog amplifier, a gain control loop of said analog amplifier receiving output signals from the A/D converter, characterized in that said analog-to-digital converting apparatus further comprises at least one comparator for making a binary comparison between an output level of the analog-to-digital converter and at least one predetermined threshold level, and means for controlling the amplifier gain with respect to time as a function of a result of said comparison, wherein the means for controlling the amplifier gain with respect to time comprises a first controllable current source for charging and discharging a first capacitor which is coupled to a gain control input of said amplifier.
 2. An analog-to-digital converting apparatus as claimed in claim 1, characterized in that said first controllable current source has a first differential stage comprising a first and a second transistor whose emitters are coupled to a first control current source, a collector of the first transistor being coupled to a supply voltage source and a collector of the second transistor being coupled to said first capacitor as well as to a second control current source, bases of the first and second transistors receiving a first differential signal corresponding to the result of said comparison, the first control current source having a current value significantly higher than that of the second control current source, so that the first capacitor is charged with a first time constant through the second current source for a first logic state of the first differential signal and, for a second logic state of the first differential signal, the first capacitor is discharged with a second time constant shorter than said first time constant through the first current source.
 3. An analog-to-digital converting apparatus having an automatic gain control circuit comprising an analog gain controllable amplifier and an analog-to-digital (A/D) converter coupled to an output of said analog amplifier, a gain control loop of said analog amplifier receiving output signals from the A/D converter, characterized in that said analog-to-digital converting apparatus further comprises at least one comparator for making a binary comparison between an output level of the analog-to-digital converter and at least one predetermined threshold level, and means for controlling the amplifier gain with respect to time as a function of a result of said comparison, wherein said analog-to-digital converting apparatus also comprises means for controlling, with respect to time, a continuous level at a signal input of the amplifier as a function of a result of a comparison produced by a second decoder of a logic level, corresponding to a continuous level standardized by at least one television transmission system.
 4. An analog-to-digital converting apparatus as claimed in claim 3, characterized in that said means for controlling the continuous level with respect to time comprises a second controllable current source for charging and discharging a second capacitor which is coupled in series to the signal input of said amplifier.
 5. An analog-to-digital converting apparatus as claimed in claim 4, characterized in that said second controllable current source has a second differential stage comprising a third and a fourth transistor whose emitters are coupled to a third control current source, a collector of the third transistor being coupled to a collector of a fifth transistor having a emitter coupled to said supply voltage source, a collector of the fourth transistor being coupled to said second capacitor as well as to a collector of a sixth transistor having an emitter coupled to said supply voltage source, bases of the fifth and sixth transistors being intercoupled, the base and the collector of the fifth transistor being coupled together so that the fifth and the sixth transistors form a current mirror circuit.
 6. An analog-to-digital converting apparatus having an automatic gain control circuit comprising an analog gain controllable amplifier and an analog-to-digital (A/D) converter coupled to an output of said analog amplifier, a gain control loop of said analog amplifier receiving output signals from the A/D converter, characterized in that said analog-to-digital converting apparatus further comprises at least one comparator for making a binary comparison between an output level of the analog-to-digital converter and at least one predetermined threshold level, and means for controlling the amplifier gain with respect to time as a function of a result of said comparison, wherein said comparator comprises a first decoder of a logic level corresponding to at least one white level of at least one television transmission system, and wherein the means for controlling the amplifier gain with respect to time comprises a first controllable current source for charging and discharging a first capacitor which is coupled to a gain control input of said amplifier.
 7. An analog-to-digital converting apparatus as claimed in claim 6, characterized in that said first controllable current source has a first differential stage comprising a first and a second transistor whose emitters are coupled to a first control current source, a collector of the first transistor being coupled to a supply voltage source and a collector of the second transistor being coupled to said first capacitor as well as to a second control current source, bases of the first and second transistors receiving a first differential signal corresponding to the result of said comparison, the first control current source having a current value significantly higher than that of the second control current source, so that the first capacitor is charged with a first time constant through the second current source for a first logic state of the first differential signal and, for a second logic state of the first differential signal, the first capacitor is discharged with a second time constant shorter than said first time constant through the first current source. 